An NOx storage-and-reduction type catalyst has been used as a catalyst for converting exhaust gas for lean-burn engine application. This NOx storage-and-reduction type catalyst stores NOx in lean atmosphere, and reduces and then converts NOx, which have been released from an NOx storage material at the time of rich spiking, by means of reducing components, such as HC, which exist abundantly in the atmosphere. The NOx storage material, which has released NOx, recovers the NOx storing capability, and stores NOx again in the subsequent lean atmosphere.
An NOx storage material comprises an alkali metal or alkaline-earth metal, and reacts with NOx to turn into the nitrates or nitrites and thereby stores NOx therein. However, the sulfur component (SO2), which arises from fuel, is included in exhaust gas. This sulfur component turns into SOx on catalyst, absorbs moisture in the exhaust gas to generate sulfuric-acid ions and sulfurous-acid ions, and then these react with the NOx storage material to generate sulfates and sulfites. Since these sulfates have extremely high decomposition temperatures compared with those of nitrates, it is difficult for the NOx storage material that has turned into the sulfates to release SOx. Consequently, there is such a problem that the NOx storage material, which has turned into the sulfates, loses the NOx storing capability and thereby the NOx converting performance of NOx storage-and-reduction catalyst lowers gradually. This phenomenon is referred to as the “sulfur poisoning.”
In order to solve this problem, an exhaust-gas converting apparatus in which an SOx absorption agent is put in place on an upstream side of an NOx storage-and-reduction type catalyst and a reducing-agent supplying means is put in place on a furthermore upstream side of the SOx absorption agent is proposed in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2006-144,624, for instance. In the same gazette, as for the SOx absorption agent that retains them in the form of stable sulfates, a sulfur storage catalyst is exemplified, sulfur storage catalyst in which an alkali component, such as K, Na, Cs and Ca, and a noble metal, such as Pt, are supported on a support that comprises alumina.
However, in such a sulfur storage catalyst on which an alkali component and a noble metal, such as Pt, are supported, there has been such a drawback that the oxidizing activities of the noble metal are lowered by means of the alkali component. Accordingly, the reaction activity of oxidizing SO2 to turn it into SOx at the time of use lowers gradually, and SO2, which has not been stored, has flowed out to the downstream side. Consequently, SOx has flowed into the NOx storage-and-reduction type catalyst that is put in place on the downstream side, and thereby it becomes difficult to prevent the sulfur poisoning.    Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2006-144,624